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This invention relates to the amplification of communications signals, and more particularly to an improvement allowing practical processing of rapid phase changes in polar modulation signals.
Many signal modulation techniques require rapid or nearly instantaneous in the phase of signals. For example, a simple binary phase shift keying (BPSK) signal requires a 180 degree phase change of the carrier signal when the data used to modulate the carrier signal changes from one binary value to another, e.g., from xe2x80x9c1xe2x80x9d to xe2x80x9c0.xe2x80x9d As a practical matter, the ability to generate and process such rapid phase changes is constrained by the finite bandwidth limitations of physical devices. In the case of polar representation signals, that is, signals expressed in terms of magnitude and phase, the generation and processing of rapid phase changes is especially challenging.
FIG. 1A illustrates an analog signal f(t) having a rapid phase change of about 180 degrees. As shown in the figure, the signal f(t) transitions from a first point 102 to a second point. The transition represents a phase change of almost 180 degrees in the signal f(t) and spans a very short period of time. Note that the phase change of nearly 180 degrees is shown here for clarity of illustration. The phase change can be less than, equal to, or greater than 180 degrees.
FIG. 1B illustrates a corresponding digital signal f(n) having a rapid phase change of about 180 degrees. Here, the signal f(n) transitions from a first point 106 to a second point 108. The transition represents a phase change of almost 180 degrees in the signal f(n) and spans very few samples of n. Specifically, only two samples, intermediate points 110 and 112, exists between the first point 106 and the second point 108. Again, the phase change of nearly 180 degrees is shown here for clarity of illustration. The phase change can be less than, equal to, or greater than 180 degrees.
FIG. 1C is a complex vector plot of the nearly instantaneous phase change in f(t). The signal f(t) can be expressed as the real part of a complex vector rotating in the complex plane, according the equations:             sin      ⁢              xe2x80x83            ⁢      wt        =                            ⅇ                      j            ⁢                          xe2x80x83                        ⁢            wt                          -                  ⅇ                                    -              j                        ⁢                          xe2x80x83                        ⁢            wt                                      2        ⁢        j                        cos      ⁢              xe2x80x83            ⁢      wt        =                            ⅇ                      j            ⁢                          xe2x80x83                        ⁢            wt                          +                  ⅇ                                    -              j                        ⁢                          xe2x80x83                        ⁢            wt                              2      
where f(t) is sin wt, w is the instantaneous rate of phase change, and the complex vector ejwt rotates about the center of a real axis Re{ejwt} and imaginary axis Im{ejwt}. The center is also termed the origin of the complex plane. Note that for clarity of illustration, the phase change in f(t) of almost 180 degrees is not incorporated into the above equations. However, the phase change is illustrated in the complex vector plot of FIG. 1C. Here, as the complex vector ejwt rotates in a counter-clockwise direction, it transitions nearly instantaneously from a first position 114, corresponding to the first point 102, to a second position 116, corresponding to the second point 104.
An angle 118 formed between the first position 114 and the second position 116 represents the phase change of almost 180 degrees. This change of phase occurs in a very short period of time. In a polar representation system, where a signal is expressed in terms of magnitude and phase, the expression of phase experiences a nearly instantaneous change corresponding to the angle 118. Such a rapid change in the value of the phase expression is associated with a correspondingly wide bandwidth. The shorter the period over which the phase change occurs, the wider the associated bandwidth becomes.
In addition, the trajectory followed by the complex vector ejwt in its transition from the first position 114 to the second position 116 can dramatically increase the severity of rapid phase change even further. Under certain conditions, the trajectory followed by the complex vector ejwt as it transitions from the first position 114 to the second position 116 is one that passes near the origin of the complex plane. The phase change experienced by the complex vector ejwt increases sharply as the trajectory nears the origin of the complex plane. Although this situation can occur in both analog and digital signal, it is more easily illustrated in the context of a digital signal. Therefore, it is explained below using the example of the digital signal f(n).
FIG. 1D is a complex vector plot of the nearly instantaneous phase change in f(n). The relationship of the signal f(n) to the complex vector plot shown in FIG. 1D is analogous to the relations already described between the signal f(t) and the complex vector plot shown in 1C. Here, a complex vector ejwn, in the form of discrete samples, rotates in a counter-clockwise direction in the complex plane. The complex vector ejwn transitions nearly instantaneously from a first position 120, corresponding to the first point 106, to a second position 122, corresponding to the second point 108.
The trajectory traced by the endpoint of the complex vector ejwn (the signal point) over time is of considerable interest in communications engineering (one end of all vectors is at the origin). For bandlimited signals, which includes nearly all signals of practical interest, the speed of the signal point along its trajectory is upper bounded. Sampled points of this trajectory are represented in FIG. 1D by intermediate positions 124 and 126, which correspond respectively to intermediate points 110 and 112 of FIG. 1B.
Should this trajectory pass near the origin, the polar coordinates of the signal point can change quite rapidly indeed. As seen in FIG. 1D, even though the direct distances between the signal points of complex vectors 120, 124, 126, and 122 respectively are nearly uniform, the angles subtended between adjacent vectors, and the magnitude changes between adjacent vectors, can change markedly. Note that the closer such a trajectory passes to the origin, the greater the associated phase change of the signal during this near approach.
FIG. 2A is a vector transition diagram of a representative bandlimited signal. Note that there are numerous transitions near to the origin, some of which transition very close to the origin. FIG. 2B is a plot of the power spectral density (PSD) of the phase change of the signal shown in FIG. 2A. Note that the PSD does not roll off very fast with increasing frequency, showing that there is a large amount of high frequency content in the phase changes of this signal. Such high frequency energy is due to the rapid phase changes whenever the signal trajectory passes nearby the origin. Representation of this type of signal directly using polar coordinates requires devices having correspondingly wide bandwidths. This xe2x80x98bandwidth expansionxe2x80x99 is a point of difficulty in the use of polar modulation. This can be contrasted with the signal shown in FIG. 6. No trajectory of this signal passes near to the origin, and the corresponding PSD of the signal phase changes shows a marked rolloff with increasing frequency.
A need exists to provide an alternative approach to direct polar modulation, such that the large phase changes of signals having trajectories passing near to the origin do not require the application of devices with large bandwidth capability.
It is important to differentiate the present invention from earlier multiple amplifier approaches, such as LINC [ref: U.S. Pat. No. 4,178,557, entitled xe2x80x9cLinear Amplification With Nonlinear Devicesxe2x80x9d (P. Henry), issued Dec. 11, 1979] and Doherty [ref: U.S. Pat. No. 5,420,541, entitled xe2x80x9cMicrowave Doherty Amplifierxe2x80x9d (D. Upton, et.al.) issued May 30, 1995]. Specifically the LINC technique, generally, decomposes the input signal into two constant-magnitude signals, and uses phase modulation techniques on both signal components to effect AM and PM on the final output signal which is the sum of the two component signals. The Doherty technique, generally, uses two amplifiers operating on the same signal, but with an offset such that when the first amplifier (sized too small to obtain improved efficiency) goes into compression, the second amplifier begins contributing to the output power. This additional power contribution must be sufficient to result in a correct output signal, which again is the sum of the two amplifier outputs. The present invention does not contain any of the above techniques.
According to the invention, a first amplified signal is produced at a first amplifier, a second amplified signal is produced at a second amplifier, and a differential signal representing difference between the first amplified signal and the second amplified signal is generated at a subtraction unit receiving the first amplified signal and second amplified signal. The differential signal may be a final amplified polar modulation signal having a final modulated amplitude and a final modulated phase.
In a specific embodiment, a control unit generates control signals controlling a first amplitude modulator providing a first amplitude modulated signal, a first phase modulator providing a first phase modulated signal, a second amplitude modulator providing a second amplitude modulated signal, and a second phase modulator providing a second phase modulated signal. The first amplitude modulated signal and first phase modulated signal are provided to a first amplifier, which produces the first amplified signal. The second amplitude modulated signal and second phase modulated signals are provided to a second amplifier, which produces the second amplified signal.